FIG. 4 is a circuit diagram of a conventional radio frequency heating apparatus. In this type of conventional radio frequency heating apparatus, commercial power supply is connected to high-voltage power supply 101 for controlling power and magnetron 102 for heating food via power plug 98 and power-supply noise filter circuit 109. Current fuse 103 for blocking current at the time of overcurrent is attached to power noise filter circuit 109.
In this type of conventional radio frequency heating apparatus, low-voltage transformer 105 for supplying predetermined voltage from commercial power supply via power plug 98 is connected to control circuit 104 for controlling the radio frequency heating apparatus. Temperature fuse 106 is inserted into low-voltage transformer 105 in order to prevent layer short that occurs by abnormal heat, typically generated by short-circuiting failure at the secondary output of low-voltage transformer 105.
Power-supply synchronization detecting circuit 107 is provided on one of secondary outputs of low-voltage transformer 105. Power-supply synchronization detecting circuit 107 obtains information on power supply phase and information on commercial frequency required for controlling the radio frequency heating apparatus by detecting power-supply frequency of commercial power supply input from power plug 98 via winding of low-voltage transformer 105. Microcomputer 99 configuring control circuit 104 uses these pieces of information for time measurement and for controlling input phase typically of relay 100. By controlling input phase of relay 100, wear of a contact of relay 100 can be minimized.
Temperature switch 108 is attached to one of power supply lines to low-voltage transformer 105. Temperature switch 108 is attached so as to detect abnormal heat generation from electric components in the radio frequency heating apparatus, and stop the operation. In FIG. 4, when temperature switch 108 is operated, power supply to low-voltage transformer 105 is blocked, and the operation of apparatus is stopped by stopping the operation of control circuit 104.
A radio frequency generator generates noise from high-voltage power supply 101 or magnetron 102 when it generates radio frequency, and this noise is transmitted to power plug 98 through internal wiring. As a result, the noise is transmitted to other electric appliance connected to the same receptacle via power plug 98. Therefore, if other electric appliance includes a microcomputer, the noise of radio frequency heating apparatus causes erroneous operation. This may reset other electric appliance, and stops power distribution. In the worst case, the power cannot be turned off due to runaway of microcomputer program. To reduce the influence of noise generated from high-voltage power supply 101 or magnetron 102 on other electric appliances, power-supply noise filter circuit 109 is provided.
Power supply noise filter circuit 109 reduces noise generated from high-voltage power supply 101 and magnetron 102 when the radio frequency heating apparatus generates radio frequency. Power-supply noise filter circuit 109 includes three types of electric components: Common-mode choke coil 120 for reducing low-band common noise, across-the-line capacitor 121 for reducing low-band normal mode noise, and line-to-ground capacitor 122 for reducing noise in both high-band common and normal modes. (For example, see PTL 1.)
In the case of the radio frequency heating apparatus in FIG. 4, charge accumulated at both ends of across-the-line capacitor 121 is discharged via primary winding of low-voltage transformer 105. Therefore, voltage between terminals of power plug 98 immediately drops if the user pulls out power plug 98, preventing electric shock.
However, if temperature fuse 106 built in low-voltage transformer 105 activates at failure of the radio frequency heating apparatus for some reason and is blocked off, a discharge path of across-the-line capacitor 121 is cut. Therefore, voltage at both terminals of power plug 98 remains, leaving the risk of electric shock at repair service.
In the same way, if temperature switch 108 is operated and temperature switch 108 is opened, the discharge path of across-the-line capacitor 121 is cut. Voltage at both terminals of power plug 98 thus remains, leaving the risk of electric shock. In addition, the same risk remains if wiring connecting power supply noise filter circuit 109 and temperature switch 108 or low-voltage transformer 105 is disconnected for some reason.
To reduce this risk, discharge resistor 124 is generally inserted in power supply noise filter circuit 109.
On the other hand, development of equipment friendly to global environment has been demanded, and legislation related to power consumption has been encouraged, in particular, with respect to daily-use electric home appliances. Standby power during non-operation is said to be 10% to 20% of total power consumption, and this is gaining attention. With consideration to the practical use state of radio frequency heating apparatuses, a power supply system for control circuit has been increasingly switched from low-voltage transformer to switching power supply. This is to reduce power consumption during non-operation time, i.e., standby time, rather than that during actual cooking time when radio frequency heating and heater are used.
FIG. 5 is a circuit diagram of a radio frequency heating apparatus using switching power supply. The circuit diagram of the radio frequency heating apparatus shown in FIG. 5 further reduces power consumption by minimizing load of control circuit when the apparatus is not needed. (For example, see PTL 2.) In FIG. 5, commercial power supply input from power plug 98 is connected to high-voltage power supply 101 for controlling power and magnetron 102 for heating food via power-supply noise filter circuit 123. Current fuse 103 for blocking current at the time of overcurrent is attached to power-supply noise filter circuit 123.
Unlike that in FIG. 4, switching power supply 125 is connected to power-supply noise filter circuit 123 in the radio frequency heating apparatus in FIG. 5. Output of switching power supply 125 controls input phase of relay 100. In the case of switching power supply 125, commercial power supply once smoothed by diode 127 and capacitor 128 is converted to power by a switching element and transformer. Accordingly, information on commercial frequency at the secondary side of low-voltage transformer 105 cannot be detected, unlike low-voltage transformer 105 in FIG. 4. Therefore, information required for controlling power-supply phase or radio frequency heating apparatus at commercial frequency is detected by providing power-supply synchronization detecting circuit 126 adopting a system including a photo coupler.
Even in the case of radio frequency heating apparatus using this switching power supply 125, temperature switch 108 is attached to one of supply lines of switching power supply 125. Accordingly, also in the radio frequency heating apparatus shown in FIG. 5, discharge resistor 124 inserted into power-supply noise filter circuit 123 secures the discharge path of across-the-line capacitor 121 even if temperature switch 108 is operated and becomes open. Voltage thus does not remain at both terminals of power plug 98, eliminating the risk of electric shock. This is the same when wiring that connects power-supply noise filter circuit 123, temperature switch 108, and switching power supply 125 is disconnected for some reason.
The radio frequency heating apparatuses shown in FIGS. 4 and 5 consume power in low-voltage transformer 105 that is power source for control circuit 104, switching power supply 125, or power-supply synchronization detecting circuits 107 and 126. Furthermore, discharge resistor 124 that also functions as a discharge circuit during normal use is needed in order to suppress voltage remaining in terminals of power plug 98 even if temperature switch 108 attached as a safety device or temperature fuse 106 built in low-voltage transformer 105 is operated in the radio frequency heating apparatuses in FIGS. 4 and 5. Accordingly, further extra power is consumed.    PTL 1 Japanese Patent No. 2864879    PTL 2 Japanese Patent No. 3397197